From common practice drive-trains with an automated transmission and a drive aggregate are known, such that the automated transmission provides at least two starting gears for a forward driving direction. Such an automated transmission is preferably in the form of a group transmission wherein the group transmission comprises a main transmission and in addition a splitter group and/or a range group. The automated transmission comprises a plurality of shifting elements, such that depending on the gear engaged a first number of the shifting elements of the automated transmission are closed and a second number of them are open. In addition, a drive-train with an automated transmission and a drive aggregate comprises a starting clutch, which can be a transmission-internal or a transmission-external starting clutch. When starting, such a starting clutch is operated with slip in order to determine, during the starting process, the torque transmitted by the starting clutch by virtue of its slip.
From common practice it is also known that during crawling, a drive-train with an automated transmission, a drive aggregate and a starting clutch can be operated in the forward driving direction at a crawling speed. For crawling, a starting gear for the forward driving direction is engaged in the transmission and the starting clutch is closed fully so that it is not slipping. The drive aggregate is operated in a rotational speed regulated manner. This then produces a crawling speed that depends on the speed of the rotational speed regulated drive aggregate and on the gear ratio of the starting gear engaged in the transmission.
During such crawling with a rotational speed regulated drive aggregate, a closed and therefore non-slipping starting clutch and an engaged starting gear, the vehicle can be moved at a constant, relatively slow crawling speed without actuating the gas or accelerator pedal, i.e. without having to alternate between the gas pedal and the brake pedal.
Then, when with the starting gear engaged, the starting clutch closed and not slipping and the rotational speed regulated drive aggregate the crawling speed is too high, for example in a traffic jam, and the driver wishes to reduce it, the driver has until now had to actuate the brake pedal, whereupon the crawling function is automatically terminated, so that thereafter the crawling function has to be reactivated by a control system or by the driver. This is inconvenient, so that crawling with a closed, non-slipping starting clutch, an engaged starting gear and a rotational speed regulated drive aggregate has not been regarded as fully acceptable by drivers.
Consequently a method is needed for operating a drive-train, with the help of which, while crawling with a closed, non-slipping starting clutch, an engaged starting gear and a rotational speed regulated drive aggregate, the crawling speed can be adapted by the driver without the driver having to terminate crawling by actuating the brake pedal.
DE 102 23 464 A1 concerns a method for controlling and/or regulating an automated clutch and/or an automated transmission of a motor vehicle, in which a tendency to crawl is established by a crawling strategy, such that the crawling strategy can be changed by the driver of the vehicle, namely when the driver actuates at least one key of a cruise control. A cruise control is also known as a speed regulating unit. In this prior art crawling is understood to mean a condition in which the automated clutch is operated with slip. Thus, DE 102 23 464 A1 does not relate to crawling with a closed, non-slipping starting clutch, an engaged starting gear and a drive aggregate with rotational speed regulation.